This invention relates to a technical art for achieving the reliable connection of a bonding wire in a semiconductor device.
Silicone resin is an organosilicon compound which has a basic formula where silicon having an organic group combines alternately with oxygen. The silicone resin has been used generally as an insulating material for sealing power semiconductor devices or the like, because of its little cure-shrinkage and excellent hermetic performance.
FIG. 12 is a sectional view showing an interior structure of a conventional semiconductor device sealed with a silicone resin. An insulating substrate 2 is mounted on a metal base plate 1 through a solder layer 3b. A semiconductor element 4 is mounted on the insulating substrate 2 through a solder layer 3a. A resin case 5 with a top opening is bonded to the base plate 1 and an electrode 6 is fixed in the vicinity of a sidewall of the resin case 5. A bonding wire 7 is in the form of an arch for example and connects electrically the semiconductor element 4 with the electrode 6.
A gel sealing resin member 8 is formed of silicone resin and covers the insulating substrate 2, the semiconductor element 4, the electrode 6 and the bonding wire 7. A lid 9 made of glass-epoxy resin covers a top opening of the case 5 and shields the sealing resin member 8 from the air. The base plate 1, the case 5 and the lid 9 constitute a housing.
When vibrations of e.g. 10G or so in magnitude were applied periodically in order to evaluate the reliability of a semiconductor device with the structure mentioned above, failure in connection was found in the bonding wire 7. The failure in connection results from fatigue fracture of the bonding wire 7. The endurance of a bonding wire 7 against vibrations largely relies upon a laying site and shape of the bonding wire 7.
The vibration of the semiconductor device makes the gel sealing resin member 8 jolt within the housing. Then a tensile stress with a magnitude corresponding to the jolting amplitude of the sealing resin member 8 is applied on the bonding wire 7 which is in close contact with the sealing resin member 8. The amplitude of the sealing resin member 8 is small in the vicinity of the sidewall of the case 5 but large in a central region of the case 5, particularly in the central region near the top surface of the sealing resin member 8. Because tensile stress is nearly proportional to the amplitude of the sealing resin member 8, the maximal tensile stress is applied on an area near the summit of an arch in the bonding wire 7.
The tensile stress applied on the bonding wire 7 is supported by a reaction force at bonded points of the bonding wire 7 with the electrode 6 or semiconductor element 4. Because the tensile stress on the bonding wire 7 is nearly proportional to the contact length of the bonding wire 7 and the sealing resin member 8, larger tensile stress is applied on the bonded point, as the bonding wire 7 becomes longer.
Moreover, the bending stress applied on the bonding wire 7 is focused on the part where the bonding wire begins to describe an arch from a bonded point, because the bonding wire 7 which is in the form of an arch is constrained on the bonded points with the electrode 6 and semiconductor element 4. The bending stress increases as the height of an arch summit from the bonded point becomes larger.
Because the tensile stress due to the vibration acts periodically on the vicinity of the bonded point on which bending stress is focused, fatigue fracture of the bonding wire 7 proceeds at this area. The bonding wire 7 with longer length and larger height of arch, disposed in the central region of the case 5 where the sealing resin member 8 jolts greatly in amplitude, will have less number of endurance cycles to a fracture.
Moreover, in a case where a case 5 has a step in the vicinity of its sidewall, the amplitude of sealing resin member 8 changes greatly before and after the step. Thus, the bonding wire 7 will have a focus of stress in the vicinity of the step and cause fractures readily.
There is an exemplary disclosure of a conventional semiconductor device that is designed to prevent fracture of a bonding wire 7 in Japanese Patent Laid-Open No. 246430/1997. This device employs, on a surface of gel member, a protection film made of a material having an elastic modulus greater than the gel, e.g. epoxy resin, to suppress vibration of the gel. This method requires the formation of a protection film in addition to the gel member. Accordingly, the process for resin curing is carried out twice and results in low productivity. Furthermore, because linear expansion coefficient is different between the material for forming a protection film and the material for forming a case, the protection film peels off readily from the close-contact face of the case. In the worst case, if an arch summit of a wire is arranged around a center of the semiconductor device where the amplitude of gel is maximal, the protection film cannot sufficiently restrain the vibration of the wire.
Japanese Patent Laid-Open No. 050897/1998 discloses another example of a semiconductor device. Epoxy resin is supplied to between a sidewall of the case and a partition plate provided in the vicinity of a wire bonded point, to cover the vicinity of the bonded point with a cured epoxy resin. Although the covering relaxes the stress focused in the vicinity of the wire bonded point, the additional process of supplying/curing the epoxy resin lowers the productivity. Furthermore, because the state of stress greatly changes in the region beyond the partition plate, the wire is readily broken around right above of the partition plate. In addition, it is impossible to prevent the gel member from jolting at around an arch summit of the wire where the maximal stress takes place due to gel vibration.
Furthermore, Japanese Patent Laid-Open No. 107147/1988 discloses a semiconductor device having vibration preventing members which are arranged to cross lengthwise and breadthwise each other on the backside of a package cap. When a semiconductor device is equipped with a control board inside which supplies control signals to a semiconductor element, the effect of the vibration preventing members considerably decrease, because the control board is located below the vibration preventing members. Consequently, it is practically impossible to provide a control board within the semiconductor device. Furthermore, it is needed to achieve gel curing after gel injection is made followed by the cap fixing, or to achieve gel injection/curing after the cap fixing is made. Accordingly, it is difficult to confirm that the cured gel is in good state or not. Unless the trouble on the gel such as cure impediment can be determined, productivity will be hindered.
Accordingly, it is an object of the present invention to provide a semiconductor device at low cost which works with high reliability even if used in a situation to cause vibration.
A semiconductor device according to the present invention comprises: a semiconductor element provided within a housing having an electrode; a bonding wire connecting the electrode with the semiconductor element; a sealing resin member covering the electrode, the semiconductor element and the bonding wire; and a sheet member fixed in the housing, arranged out of contact with the bonding wire and buried in the sealing resin member. Because the sheet member acts as a vibration reducing member which restrains the vibration of the sealing resin member, the bonding wire is connected with higher reliability. Therefore the semiconductor device attains an increase in lifetime.
The sheet member may have an opening portion, wherein the opening ratio is desirably not less than 20% and not more than 80%. When the opening ratio falls within the range above, the sheet member firmly adheres to the sealing resin member. The opening portion of the sheet member can be arranged above a bonded point of the electrode with the bonding wire or above a bonded point of the semiconductor element with the bonding wire. The sealing resin member thermally swelled escapes upward through the opening portion. Because the sheet member may have a shape extending along a shape of the bonding wire, the clearance to the bonding wire can be reduced.
In the above area of the sealing resin member may be provided a space member which is not less than 10% and not more than 100% in volume of the sealing resin member. When the sealing resin member thermally swells, the space member restrains the increase of pressure inside the semiconductor device.
A control board may be arranged above the sheet member. The sheet member also restrains the vibration of the control board buried in the sealing resin member. The sealing resin member may be formed of silicone resin.
Moreover, close contact of the sheet member may be made with the upper surface of the sealing resin member. In this case, the sheet member may comprise an upper member and a lower member, and the bottom surface of the lower member contacts the upper surface of the sealing resin member closely.
The lower member and the upper member are easy to bond because they may be fixed together with an adhesive layer. Clearance to the bonding wire is reduced because the lower member may have a shape which extends along a shape of the bonding wire.
Furthermore, the semiconductor device according to the present invention may have a pillar member in place of the sheet member. The pillar member is out of contact with the bonding wire and fixed on an insulating substrate, and further, buried in the sealing resin member. Because the pillar member acts as a vibration reducing member which restrains the vibration of the sealing resin member, reliability of connection with the bonding wire increases. Therefore the semiconductor device attains an increase of lifetime.
The clearance from the pillar member to the bonding wire can be decreased, because the pillar member may have a shape extending along a shape of the bonding wire. When the pillar member made of aluminum or copper is bonded to a copper pattern which is interposed between the semiconductor element and the insulating substrate, an electroconductive path is formed.